There are three different screening units that I will talk about in this article, beasts, hellions, and reavers.
Beasts: In a screening format, I recommend taking minimum of 3 beast masters and 6 razor wing flocks for a tiny point cost of 138 points. The advantages this unit has over the other two is that it will be much more durable and much better in counter charging close combat threats. The disadvantages is, as there are no models yet we do not know how tall they will be or whether or not razorwings will be on flying bases. This is important due to whether or not they will be able to obscure vehicles behind. Also moving them up with your vehicles may be rather tricky, considering that beasts will have to run in order to screen your vehicles.
Out of these three units I believe that each is viable to screen. However, they each excel depending on how close to the enemy they are. Beasts excel at being very close, hellion excel where they can pump out some shots, and reavers excel at longer ranges.
Screening: It's uses
Screening, in nearly all aspects of the game, is extremely helpful. It makes your vehicles more survivable by creating cover. It creates space in which deep striking units can't land. And above all it makes nearly every unit holding a short ranged gun quake with fear.
Units: Beasts: 3x beastmasters with 6x razorwing flocks 138pts
4x trueborn, 4x blasters, Venom with extra SC and nightfield 178pts
5x warriors, 1x blaster, venom with extra SC and nightfield 145 pts
This type of setup will work wonders against armies that rely on melta guns and multimeltas. If you can fill the 5 inches in front of the venoms with the footprint of the beasts, the enemy won't be able to get in range with his melta guns, and will not be able to get in "melta range" with his multi meltas. This setup also can be completely self sufficient. You have 5 blasters to blow up tanks with, 24 splinter cannon shots to weaken squads, and close combat unit to take charges that the vehicles would normally take. All the while, you have a 33 wound unit that obscures the vehicles behind, making them twice as durable. Correct, blast templates will hurt, but that's why you have proper target selection right? ^^
Why it's important for dark eldar:
Screening is important for everyone, however it is especially important for dark eldar. Dark eldar have the most expensive/fragile transports in the game. This is for balence purposes, seeing that what is inside of those transports is usually much more potent than other races. However, once you begin to nullify how fragile the transports are, you can gain a massive advantage.